The present invention relates to hydrogenated amorphous silicon alloys useful in the fabrication of photovoltaic devices and other semiconductor devices.
It is desirable to include carbon or germanium atoms in hydrogenated amorphous silicon alloys in order to adjust their optical bandgap. For example, carbon has a larger bandgap than silicon and thus inclusion of carbon in a hydrogenated amorphous silicon alloy increases the alloy's bandgap. Conversely, germanium has a smaller bandgap than silicon and thus inclusion of germanium in a hydrogenated amorphous silicon alloy decreases the alloy's bandgap.
Similarly it is desirable to incorporate boron or phosphorous atoms in hydrogenated amorphous silicon alloys in order to adjust their conductive properties. Including boron in a hydrogenated amorphous silicon alloy creates a positively doped conductive region. Conversely, including phosphorous in a hydrogenated amorphous silicon alloy creates a negatively doped conductive region.
Hydrogenated amorphous silicon alloy films are prepared by deposition in a deposition chamber. Heretofore, in preparing hydrogenated amorphous silicon alloys by deposition in a deposition chamber, carbon, germanium, boron or phosphorous have been incorporated into the alloys by including in the deposition gas mixture carbon, germanium, boron or phosphorous containing gases such as methane (CH.sub.4), germane (GeH.sub.4), germanium tetrafluoride (GeF.sub.4), higher order germanes such as digermane (Ge.sub.2 H.sub.6), diborane (B.sub.2 H.sub.6) or phosphine (PH.sub.3). See for example, U.S. Pat. Nos. 4,491,626, 4,142,195, 4,363,828, 4,504,518, 4,344,984, 4,435,445, and 4,394,400. A drawback of this practice, however, is that the way in which the carbon, germanium, boron or phosphorous atoms are incorporated into the hydrogenated amorphous silicon alloy is not controlled. That is, these elements are incorporated into the resulting alloy in a highly random manner thereby increasing the likelihood of undesirable chemical bonds.